This invention relates to a post binder of the type used for catalogs and reference works and the like in which a relatively thick stack of replaceable leaves is held by a post mechanism having a plurality of straight posts extending through the stack. There is a basic need for a binder of this type which will close to a condition in which it is little thicker than the thickness of the stack of leaves and will open to a condition in which the leaves are loose and separated from each other so as to lie open for convenient examination.
There are available on the market a number of extensible-post binders in which the backbone of the cover structure consists of two or more sliding parts which can be pushed to a collapsed condition when closed and pulled to an expanded position when opened. These require a relatively complex structure, are not held collapsed when closed, and in general are awkward and unattractive. See, for example, U.S. Pat. No. 2,784,719 issued Mar. 12, 1957 to C. J. Panfil. Other extensible-post binders on the market rely primarily on the post structure to support the covers and either have no backbone between the covers or have merely flexible facing between the covers which flexes to allow the post structure to be manually expanded and collapsed. See, for example, U.S. Pat. No. 2,253,039 issued Aug. 19, 1941 to G. Lewis.
The present invention provides an improved binder in which the post mechanism is automatically expanded as the binder is opened and automatically collapsed as the binder is closed. The binder is especially adapted to be made with a one-piece cover structure formed of stiff plastic sheet material, and which when closed has an especially neat and attractive appearance with no exposed rivets or the like.